Coupling light of a semiconductor laser diode into an optical fiber is a central problem within the field of optical networks, in particular when high power transmission/coupling is desired. Due to increasing channel density in DWDM (Dense Wavelength Division Multiplexing) long haul networks, and the power requirements at elevated temperatures in metro networks, maximizing the laser diode's operating light output power is a primary design criterion. The useful operating power is mainly limited by a “kink” in the L-I curves, i.e. the light output over current curves, indicating a beam steering in lateral direction. The occurrence of such a kink is influenced by the real refractive index step, the gain profile as well as spatial hole burning and local heating in the laser diode. Depending on the device structure, the laser diode suffers at a certain power level from the resonance between the fundamental mode and higher order modes in lateral direction. This has been shown by J. Guthrie et al in “Beam instability in 980 nm power lasers: Experiment and Analysis” in IEEE Pot. Tech. Lett. 6(12), 1994, pp. 1409-1411. Generation of higher order modes is highly undesirable since efficient laser to fiber coupling is only possible with the fundamental mode.
Since weakly guided semiconductor devices like ridge waveguide (RWG) laser diodes are preferred for high power applications, as shown by B. E. Schmidt et al in “Pump laser diodes”, Optical Filter Telecommunications IVA, Editors: Kaminov and Li, Academic Press, 2002, ISBN 0-12-395172-0, pp. 563-586, an improvement in RWG designs appears highly desirable.
Bowler U.S. Pat. No. 6,141,365 describes a semiconductor laser with a kink suppression layer. Reportedly, the latter limits the establishment of higher order lateral modes and thus increases the kink power of the device. Bowler also discloses disposing an optical layer along the optical axis of an RWG laser on both sides of the laser's ridge. However, shape and size of this kink suppression layer is essentially determined by the photoresist mask used to form the ridge. Bowler does not address utilizing the kink suppression layers shape, thickness, and/or material for any particular purpose apart from general kink suppression. Also, the lasers described by Bowler have output powers of no more than 200 to 300 mW which is insufficient for many of today's technical applications.
Thus, it is a general object of this invention to devise a reliable design for a high power RWG laser diode which in particular provides a stable light output under all operating conditions and a sufficiently long life of such laser diodes. Hereinbelow, the term “high power” is used for an optical output power approximately 1 W. Laser diodes with 918 mW linear kink-free power have been realized with a design according to the present invention.
It is a further primary object of this invention to provide an advantageous and economical manufacturing method for a novel high power RWG laser diode, allowing reliable mass production of such laser diodes.
It is a more specific object of this invention to provide a RWG laser diode design optimally suited for realizing laser diodes with kink-free output powers in the 1 W region, and an increase of about 25% in median linear power (taken over about 700 devices) compared to a standard design.